1. Technical Field
This invention relates generally to the design and manufacture of aluminum containers as are commonly used for containment of beverages, and in particular to an improved bottom structure and process for manufacturing the improved bottom structure.
2. Background
Two piece aluminum containers have been used extensively for distributing beverages such as beer, carbonated soft drinks and other beverages such as tea. A large portion of the cost of delivering beverages to consumers in containers is the material cost. This has led to ongoing efforts in the industry to reduce the amount of material used in containers. The widespread use of such containers makes it possible to achieve large savings with what would otherwise appear to be a very small reductions in material requirements. These efforts are referred to within the industry as "lightweighting." Efforts to lightweight aluminum containers have focused on four areas: the bottom profile, the side wall, the neck area and the container lid.
Light weighting of the bottom profile generally involves either changing the geometry of the bottom as it is manufactured during the bodymaking process, modifying the bottom profile shape after the bodymaking process, or some combination of the two. Despite the efforts to lightweight aluminum containers, the need for a strong bottom has required substantial thickness be retained in the bottom to achieve required strengths. If the container bottom is not sufficiently strong the central dome area will be forced downward by the internal pressure of the contents until it reverses shape. This is referred to as dome reversal. As a practical matter, the dome on all aluminum containers manufactured for commercial use today will reverse at some finite pressure. The pressure at which the dome reverses is one criteria which is used to measure the strength of a particular bottom profile. This pressure is referred to as the "dome reversal pressure" or DRP. Design changes that increase the dome reversal pressure are the equivalent of design changes that permit light weighting of the container, since material thickness container be reduced while maintaining the required dome reversal pressure.
The bodymaking process includes the process steps of drawing and ironing. The bottom profile of a container is typically formed as the last step, in a pressing process that draws material to the required shape and dimensions. The bottom profile is formed when punch nose tooling, located on the bottom end of a punch on the interior of the container blank, is forced against a dome plug and outer retainer located on the opposite side of the container blank. The profile formed in the aluminum sheet feed stock will thus conform to match the intersection between the punch nose tooling, outer retainer, and the dome plug. Typically, the punch nose tooling has a coining surface which defines the point in the process where there is continuous contact between the dome plug tooling, the metal of the container blank, and the punch nose tooling.
During the creation of the bottom profile, the thickness of the aluminum in the center of the bottom is not significantly changed. Therefore, the thickness required for adequate bottom strength must be the thickness of the feed stock. The inability to improve the strength of the bottom creates a situation in which the bottom profile requirements determine the minimum acceptable feed stock thickness. Improvements in the side wall and neck area which could permit the use of thinner feed stocks cannot be implemented unless an improved bottom profile has been identified and a process for manufacturing containers with the improved profile defined. Thus, there remains a need for improved container bottom profiles and manufacturing methods therefore.
One known method of improving the strength of a bottom profile is to reform either the outside or inside area around the nose. U.S. Pat. No. 5,105,973, describes an inside reform process for improving the strength of the container. Reforming of the outside is also known.
Another limitation on known processes is that the inside diameter of the bottom profile is formed in a drawing process. In prior processes, a dome plug has been used to form a substantially linear inner leg portion between the nose and the dome. The inner leg is formed when the dome plug and opposing tooling known as the punch nose are pressed together with a metal container blank between them. The process requires sufficient clearance to allow the metal to flow between the opposing tooling without shearing and tearing. As a result of the clearances used in prior art processes, the inner leg portion, though linear, will angle slightly outwardly at the bottom, away from the longitudinal central axis 116 of the container when the dome plug tooling is removed. This occurs because the aluminum of the bottom is formed from the end of the punch nose tooling to the top of the dome plug which has a smaller diameter than the inside diameter of the inner leg portion of the punch nose tooling. The difference in diameter causes the inner leg to slant slightly toward the inner axis at the top.